Conventionally, an exposure device for producing an integrated circuit by transferring fine circuit patterns on a wafer is widely utilized in photolithography technology. With the trend toward a higher integration and higher functionality of an integrated circuit, downsizing of the integrated circuit is advanced. An exposure device is hence required to form an image of a circuit pattern on a wafer surface with high-resolution at a long focus depth, and shortening of the wavelength of an exposing source is being advanced. Deep ultraviolet such as KrF excimer laser (wavelength: 248 nm) and ArF excimer laser (wavelength: 193 nm) is going to be employed as the exposing source in place of the conventional g-ray (wavelength: 436 nm), i-ray (wavelength: 365 nm), and the like.
Synthetic silica glass has mainly been employed in an optical member of an exposure device including an exposure device employing deep ultraviolet as an exposing source for the reasons that, for example, the synthetic silica glass is transparent over the wavelength region in a wide range of from near infrared region to deep ultraviolet region, has extremely small thermal expansion coefficient and therefore has excellent dimensional stability, and does not almost contain metal impurities and therefore has high purity.
A glass having high OH group content, or synthetic silica glass having high purity having been subjected to hydrogen impregnation treatment has been used as an optical member for deep ultraviolet (for example, see Patent Document 1). This is a technique that, by increasing Si—OH contained in a glass, when a bond of SiO2 is damaged by deep ultraviolet, a repairing action on the damage by Si—OH is increased, thereby attempting to maintain durability (transmission characteristics).
Conventionally, an optical member for deep ultraviolet in which Si—OH is increased to thereby increase repairing action has been produced and used. However, it came to be known that synthetic silica glass having high OH group content is not preferred in the view points of density change induced by deep ultraviolet irradiation and durability to birefringence change induced by polarized deep ultraviolet irradiation. The reason for this is that in the case that synthetic silica glass has been irradiated with deep ultraviolet, structure change occurs in the synthetic silica glass due to the presence of OH groups.
The phenomenon that densification of synthetic silica glass occurs with deep ultraviolet irradiation is also called “compaction”. The compaction associated with deep ultraviolet irradiation causes unfavorable change on optical characteristics in the synthetic silica glass, such as increase in refractive index and occurrence of birefringence. Therefore, the compaction must be reduced as possible.
Some methods have conventionally been proposed as a method of decreasing OH group content. For example, a method of doping a synthetic silica glass with fluorine is proposed. Fluorine has an action to replace with OH group, and therefore easily decreases the OH group content. The fluorine further has an effect of decreasing viscosity coefficient of a glass. Therefore, distorted bond of Si—O—Si bond network is easily relaxed by heat treatment. In view of the above reasons, it has been considered that the method of doping a synthetic silica glass with fluorine is effective as a means of increasing durability to deep ultraviolet.
On the other hand, fluorine in an amount of several hundred ppm or more is required to be doped in order to impart sufficient deep ultraviolet durability to a synthetic silica glass by this method. However, the fluorine has an action of greatly decreasing refractive index of a glass, and in the case that distribution of fluorine doped is not uniform, striae and refractive index distribution occur. Therefore, unless this problem is solved, it is difficult to apply the synthetic silica glass doped with fluorine to an optical member for an exposure device which requires suppression of striae and high refractive index homogeneity. Furthermore, in the case that the synthetic silica glass doped with fluorine is used as an optical member for deep ultraviolet, there are some cases that a part of fluorine is liberated from the glass with the progress of irradiation with deep ultraviolet and is released as high-reactive F2, and thereby, a device having the synthetic quart glass provided therein may be adversely affected. Thus, the synthetic silica glass doped with fluorine has various problems to be solved, for use as an optical member for deep ultraviolet.
In view of the above, it is considered that a synthetic silica glass in which content of OH group is small, defect of absorbing deep ultraviolet and precursor structure of the defect are not present, and the defect and its precursor structure do not formed even though heat treatment and irradiation of deep ultraviolet are continued, is preferred as an optical member for deep ultraviolet.
Patent Document 2 discloses an optical member for ultraviolet containing a synthetic silica glass obtained from a high purity silicon compound by heating a porous synthetic silica glass body obtained by depositing soot produced by a soot process of flame pyrolyzing the silicon compound, in which impurities other than OH group are not substantially contained, difference between the maximum value of a fictive temperature in the glass and the minimum value thereof is 50° C. or less, and transmission of ultraviolet having a wavelength of 157 nm is 60% or more in a optical path length of 10 mm. The optical member is an optically stable member having good transmission of deep ultraviolet and free of compaction associated with deep ultraviolet and change of light transmittance, and is considered that light absorption due to heat treatment at high temperature and irradiation with deep ultraviolet does not occur, and homogeneity is not deteriorated. In the optical member for ultraviolet, it is considered that OH group content is from 1 to 70 ppm by mass, Cl concentration is less than 1 ppm by mass, metal impurity concentration of each element is less than 10 ppb by mass, and the total amount of metal impurity concentrations is 50 ppb by mass or less.